The main function of the easy to pull lid and bottom lid expansion ring is to increase the strength of the can lid and ensure the sealing structure of the can. Due to the rapid expansion of the residual air volume inside the can during the sterilization process, the volume increases and the pressure inside the can increases significantly. In order to prevent leakage caused by the deformation of the can lid due to expansion, and to ensure that the can lid can return to its original state after cooling, in addition to using back pressure measures during sterilization and cooling, heating and vacuuming during sealing to increase the internal stress of the can, it is also necessary to press a certain form of expansion ring pattern on the can lid to ensure that it has sufficient structural strength and rebound ability.

From long-term production practice, it is known that the form of the expansion ring of the easy to pull lid and bottom lid is related to the canning variety, content organization, and vacuum degree of the can. Generally speaking, the contents of the can are formed into blocks, with a small top gap and low vacuum degree, such as luncheon meat and products with bones. The strength of the expansion ring pattern should be greater. And generally, canned foods with more soup, larger top gaps, and higher vacuum levels inside the can, such as clear vegetables, should have good rebound ability in terms of their expansion loops.